In the present power supplies, a synchronous rectifier as they are commonly known utilizes a transformer to drive the transistors of the synchronous rectifier so as to obtain an effective filtering and rectifying conversion of the AC input signal. However, the conventional synchronous rectifier utilizes diodes to be a rectifier. Furthermore, it is difficult to apply a synchronous rectifier which is composed of diodes to a low voltage power supply because the forward voltage drop of the diodes generates more than 50% power dissipation. Therefore, a high efficiency and low voltage power supply generally utilizes MOSFETs instead of diodes in order to reduce the power dissipation of the forward voltage drop of the diodes.
Please refer to FIG. 1. It is a schematic diagram of MOSFETs used as a synchronous rectifier according to the prior art. As shown in FIG. 1, the synchronous rectifier 10 includes a transformer 11, two MOSFETs 12, 13, a filtering inductor 14, and a filtering capacitor 15. The transformer 11 also includes a primary winding 111, two secondary windings 112, 113, and two auxiliary windings 114, 115. Two auxiliary windings 114, 115 are utilized to forward-biase the MOSFET 12, 13.
The drawback of the conventional method is that the efficiency of the synchronous rectifier will be reduced and the conduction loss of the MOSFETs will be increased when the synchronous rectifier operates at light load and the operation of the synchronous rectifier at light load is the same to that at heavy load.
It is therefore attempted by the applicant to deal with the above situation encountered with the prior art.